This invention relates to a method of producing a cross-linked polyolefin foam in a continuous process without using a moisture cross-linking step after processing.
Some thermoplastic polymers, such as polyethylene, are not suitable for higher temperature applications because they soften and lose their physical properties. The cross-linking of polyethylene changes the polymer from a thermoplastic to a thermoset, resulting in a more durable, heat resistant material with improved mechanical properties. Applications of cross linking polymers include extrusion molding, pipe molding, wire coating and foams.
There are three primary techniques used to cross-link polyolefins: (1) chemical cross-linking using peroxides (2) electron beam irradiation and (3) moisture cross-linking. Chemical and irradiation cross-linking techniques require considerable capital investments, large manufacturing areas, and complex equipment. U.S. Pat. No. 4,186,068 describes a method of producing high density cross-linked polyethylene foams via a radiation method. Closed foam cells of uniform structure were created. Moisture cross-linking was first developed by Dow Corning, known as the Sioplas process (British Patent No. 1,286,460). Cross-linking takes place using silane chemistry and moisture with moderate costing equipment. Silane substituents are introduced onto the polymeric backbone via melt grafting in the presence of a peroxide catalyst. Silane groups will hydrolyze in the presence of moisture to give silanols (—Si—OH) and then condense to siloxanes (—Si—O—Si—). The siloxane structure will become a crosslink between the polyethylene chains. U.S. Pat. No. 3,646,155 describes a process for obtaining a composition of a cross-linkable polyethylene by extruding a blend of polyethylene, a peroxide and a vinylalkoxysilane.
A disadvantage of the moisture cross-linking method is its slow reaction kinetics. It is often necessary to cure the plastic in hot water or a high humidity tank at 80° C. or above for 24 to 36 hours to promote cross-linking. Several attempts have been made to reduce the cross-linking time. These include the introduction of a catalyst to a silane grafted polymer (Japanese Patent No. A-57-208006, Japanese Patent No. A-62-106947) and a method where the contact with moisture is enhanced (Japanese Patent No. A-60-2545520).
Over the past few decades, polymeric foams have garnered rapid growth in the plastics industry. The many benefits of polymeric foams include weight reduction, superior insulating abilities, exceptional strength to weight ratio and energy absorption (shock, vibration and sound). Foams have been prepared in a variety of polymers such as polyurethane, polystyrene, polyvinyl chloride, epoxys and polyolefins. Polymeric foams have been used in a wide variety of applications including automotive, packaging, insulation, flotation, furniture, bedding, textiles and toys to name a few. Several different processing techniques have been used to produce foams including compression molding, extrusion, injection molding, rotational molding and cast film. The preparation of silane grafted cross-linked polyolefin foams is described in U.S. Pat. No. 4,870,111, U.S. Pat. No. 5,589,519, U.S. Pat. No. 5,844,009, U.S. Pat. No. 4,591,606, and U.S. Pat. No. 6,812,262.
U.S. Pat. No. 4,870,111 describes a method of producing moldable foam beads comprising a silane-crosslinked polyolefin foam using a silane-grafted polyethylene.
U.S. Pat. No. 5,589,519 describes a method and apparatus for producing a lightly cross-linked polyolefin foam using an extruder.
U.S. Pat. No. 5,844,009 describes a method of forming low density polyethylene (LDPE) cross-linked foams with the use of physical-blowing agents and a silane-grafted polyolefin resin.
U.S. Pat. No. 4,591,606 describes a method of forming a silane-cross-linked polyolefin foam produced by extruding a mixture containing polyethylene, silane, a condensing catalyst and water acting as foaming agent.
U.S. Pat. No. 6,812,262 describes a method of preparing a silane-crosslinking polyolefin foam with the use of organic and inorganic blowing agents and a silane grafted polyolefin. Foaming is carried out immediately after heat molding without the conventional moisture operation yielding a crosslinked foam with superior heat resistance properties.